[Steely Tom]

You want “clever”, look into the original color TV design. Done initially with vacuum tubes (not thousands of transistors), and the color signal would work with the old black and white TVs. And the sets were manufactured at consumer prices.

True. No argument there.

TVs were one of the first places where phase-locked loops (PLLs) were used in a consumer product.

Color TV depended on PLL technology even more.

FM Stereo wouldn't have been possible without it.

The PLL was one of the great innovations of electronics, incredibly useful and versatile. A case where the whole was much greater than the sum of its parts, the added ingredient being cleverness.

[Myrddin]

We had an old Fisher tube based, mono FM tuner when I was 4 years old. It went with us to Hawaii where I turned 5 on the ship between San Francisco and Honolulu. The advent of stereo FM was announced. My dad purchased a Sherwood amplifier (tubes) and an FM demultiplexer (tubes). The DeMux had a 38 KHz sub-carrier circuit and an analog sum and difference circuit for the L+R (mono) and L-R (subcarrier) to accomplish 2L = (L+R) + (L-R); 2R = (L+R) - (L-R). I suspect the 38 KHz subcarrier circuit was mixing the audio down to baseband before performing the sum and difference operations. Input to the demux was from the Fisher discriminator. An old Foster-Seeley style implemented with a transformer.

[cymbeline]

“The PLL was one of the great innovations of electronics”

I’m barely a hardware guy, mostly curious and lightweight reading.

It’s interesting to think about the clever, small, electrical and mechanical mechanisms that have been invented.

Getting back to the rotary phones. How in the world could the exchanges take those pulses and connect thousands of callers to their destinations all at once with bunches of relays?